public class Example4_13 {
    public static void main(String args[]) {
	Circle circle = new Circle();
	circle.setRadius(196.87);
	Tixing lader = new Tixing(3,21,9);
	Student zhang = new Student();
	System.out.println("zhang计算圆的面积:");
	double result = zhang.computerArea(circle);
	System.out.println(result);
	System.out.println("zhang计算梯形的面积:");
	result = zhang.computerArea(lader);
	System.out.println(result);
    }
}

